Fiber reinforced composite materials with polymeric matrices (FRP) including carbon fiber reinforced polymers (CFRP) can be designed and constructed that have outstanding mechanical and physical properties such as low density, high tensile and torsional strength, and high modulus of elasticity or stiffness. A variety of high strength fiber materials can be used including carbon fibers, glass fibers, silicon carbide fibers, and fibers of many other oxides, carbides, and other materials. Similarly, a wide variety of polymeric materials can be used including thermosetting resins such as phenolic resins, epoxies, and many others. The fibers may be very long and positioned in specific patterns or relatively short and randomly dispersed. When long fibers are positioned in specific patterns, they can be aligned in a single direction or positioned in patterns designed to give two or three dimensional strength to the FRP. Thus the mechanical properties of the FRP structure can be tailored to the specific requirements of a component.
Unfortunately, the surfaces of a FRP have low resistance to wear including adhesive, abrasive, and erosive wear. They may also be susceptible to oxidation or other forms of corrosion, need protection from heat, not have the required optical or electrical characteristics, etc. As a result, their utilization has been limited in many applications or has required the use of metallic or ceramic inserts or sleeves in areas of contact or exposure to wear, heat, etc. For example, a bulky and expensive wear resistant sleeve must be bonded to a FRP shaft in bearing areas to prevent adhesive or abrasive wear and FRP aircraft wing or tail components must have a metallic shield bonded on leading edges to prevent erosion. If the large rolls used in paper manufacturing and printing industries could be made of FRP they would be much lighter and stiffer thus much easier and safer to handle, require less energy and time to accelerate and decelerate (due to lower inertia), and produce better products due to rigidity. Their surface, however, would not have adequate wear resistance and could not be engraved as is required for some printing applications.
A solution to many of the problems associated with the utilization of FRP would be an adherent coating with the required wear resistance or other properties. A wide variety of metallic, ceramic, cermet, and some polymeric coatings can be produced using thermal spray deposition. Many of these materials would be useful in providing wear resistance and other properties for FRP components if they could be successfully deposited on them.
The family of thermal spray processes includes Super D-Gun™ deposition, detonation gun deposition, high velocity oxy-fuel deposition and its variants such as high velocity air-fuel, plasma spray, flame spray, and electric wire arc spray. In most thermal coating processes, a metallic, ceramic, cermet, or some polymeric material in powder, wire, or rod form is heated to near or somewhat above its melting point and droplets of the material accelerated in a gas stream. The droplets are directed against the surface of the substrate (part or component) to be coated where they adhere and flow into thin lamellar particles called splats. The coating is built up of multiple splats overlapping and interlocking. These processes and the coatings they produce have been described in detail in the following: “Advanced Thermal Spray Deposition Techniques”, R. C. Tucker, Jr., in Handbook of Deposition Technologies for Films and Coatings, R. F. Bunshah, ed., Second Edition, Noyes Publications, Park Ridge, N.J., 1994, pp. 591 to 642; “Thermal Spray Coatings”, R. C. Tucker, Jr. in Handbook of Thin Films Process Technology, Institute of Physics Publishing, Ltd., London, 1995; and “Thermal Spray Coatings”, R. C. Tucker, Jr., in Surface Engineering, ASM Handbook, Vol. 5, ASM International, Materials Park, Ohio, 1994, pp. 497–509.
In virtually all thermal spray processes, two of the most important parameters controlling the structure and properties of the coating are the temperature and the velocity of the individual particles as they impact on the surface to be coated. Of these, the temperature of the particles is of greatest import relative to coating FRPs. The temperature the particles achieve during the deposition process is a function of a number of parameters including the temperature and enthalpy (heat content) of the process gases, the specific mechanisms of heat transfer to the particles, the composition and thermal properties of the particles, the size and shape distributions of the particles, the mass flow rate of the particles relative to the gas flow rate, and the time of transit of the particles. The velocity the particles achieve is a function of a number of parameters as well, and some of these are the same as those that affect the particle temperature including the composition, velocity and flow rate of the gases, the size and shape distributions of the particles, the mass injection rate and density of the particles.
In a typical detonation gun deposition process, a mixture of oxygen and acetylene along with a pulse of powder of the coating material is injected into a barrel about 25 mm in diameter and over a meter long. The gas mixture is detonated, and the detonation wave moving down the barrel heats the powder to near or somewhat above its melting point and accelerates it to velocity of about 750 m/s. The powder's rapidly heated into molten, or nearly molten droplets of material that strike the surface of the substrate to be coated and flow into strongly bonded splats. After each detonation the barrel is purged with an inert gas such as nitrogen, and the process repeated many times a second. Detonation gun coatings typically have a porosity of less than two volume percent with very high cohesive strength as well as very high bond strength to the substrate. In the Super D-Gun™ coating process, the gas mixture includes other fuel gases in addition to acetylene. As a result there is an increase in the volume of the detonation gas products which increases the pressure and hence greatly increases the gas velocity. This, in turn, increases the coating material particle velocity, which may exceed 1000 m/s. The increased particle velocity results in an increase in coating bond strength, density, and an increase in coating compressive residual stress. In both the detonation gun and Super D-Gun coating processes nitrogen or another inert gas can be added to the detonation gas mixture to control the temperature of the detonated gas mixture and hence the powder temperature. The total process is complex, and a number of parameters can be used to control both the particle temperature and velocity, including the composition and flow rates of the gases into the gun.
In high velocity oxy-fuel and related coating processes oxygen, air, or another source of oxygen is used to burn a fuel such as hydrogen, propane, propylene, acetylene, or kerosene in a combustion chamber and the gaseous combustion products allowed to expand through a nozzle. The gas velocity may be supersonic. Powdered coating material is injected into the nozzle and heated to near or above its melting point and accelerated to a relatively high velocity, up to about 600 m/s for some coating systems. The temperature and velocity of the gas stream through the nozzle, and ultimately the powder particles, can be controlled by varying the composition and flow rate of the gases or liquids into the gun. The molten particles impinge on the surface to be coated and flow into fairly densely packed splats that are well bonded to the substrate and each other.
In the plasma spray coating process a gas is partially ionized by an electric arc as it flows around a tungsten cathode and through a relatively short converging then diverging nozzle. The partially ionized gas, or gas plasma, is usually based on argon, but may contain, for example, hydrogen, nitrogen, or helium. The temperature of the plasma at its core may exceed 30,000 K and the velocity of the gas may be supersonic. Coating material, usually in the form of powder, is injected into the gas plasma and is heated to near or above its melting point and accelerated to a velocity that may reach about 600 m/s. The rate of heat transfer to the coating material and the ultimate temperature of the coating material are a function of the flow rate and composition of the gas plasma as well as the torch design and powder injection technique. The molten particles are projected against the surface to be coated forming adherent splats.
In the flame spray coating process, oxygen and a fuel such as acetylene are combusted in a torch. Powder, wire, or rod is injected into the flame where it is melted and accelerated. Particle velocities may reach about 300 m/s. The maximum temperature of the gas and ultimately the coating material is a function of the flow rate and composition of the gases used and the torch design. Again, the molten particles are projected against the surface to be coated forming adherent splats.
Many attempts have been made to directly coat FRP surfaces with thermal spray coatings. Thermal spray coatings of metallic, cermet, or ceramic compositions usually do not adhere at all or spall when only a small amount of coating has been deposited. In most thermal spray coating applications, the surface to be coated must be roughened to provide adequate bonding. Roughening is usually done by grit blasting the surface. Grit blasting or some other forms of roughening FRP surfaces leads to unacceptable erosion of the polymeric matrix and fraying of the fibers. The later, in particular, leads to a rough and porous thermal spray coating. This and other problems were found in attempting the method of Hycner in U.S. Pat. No. 5,857,950 for example. Hycner teaches grit blasting the surface of a CFRP fluid metering roll (an anilox roll used in printing) and then thermal spraying a layer of zinc, nickel-20 chromium, or mixture of aluminum bronze plus 10 polyester at a negative rake angle of 11½ to 13½ degrees. A ceramic coating is then applied over the first layer. The ceramic coating is subsequently finished and engraved. This process has been found to be unacceptable because of poor bond strength with some of the specified first layer coatings and other production problems and because of substantial imperfections in the coating. Many other attempts to use thermally sprayed metallic undercoats have also failed. Even attempts to deposit an undercoat of a polymeric material by thermal spraying were only marginally successful in laboratory experiments and in production were difficult to reproduce in a reliable manner.
An alternative method has been taught by Habenicht in EP 0 514 640 B1. Habenicht first creates on the surface of a CFRP a layer that consists of a mixture of a synthetic resin that bonds to the CFRP and a particulate material. After this layer is cured the surface is partially removed to expose the particulate material. The particulate material must be capable of chemical bonding to the outer coating material that is thermally sprayed on the first layer. The particulate materials and the outer thermal spray coating material are selected from variety of metals and ceramics. While this method has met with limited success, the mixture of synthetic resin and particulate material may not bond well to the CFRP and tends to form balls of material on the surface, thus being unsuitable for commercial production.
Several other techniques of preparing the surface of a CFRP surface for thermal spray coatings have been described by E. Lugscheider, R. Mathesius, G. Spur, and A. Kranz in the Proceedings of the 1993 National Thermal Spray Conference, Anaheim, Calif., 7 to 11 Jun. 1993. One method appears to be similar to that of Habenicht, but the most successful method appeared to be one in which a three dimensional wire mat was laminated into the polymeric composite. The surface was then grit blasted to expose the wire and a thermal spray coating applied. This technique would be very expensive to use in industrial applications and would tend to yield a very rough surface on the thermal spray coating.
Thus the current state of the art is such that there appears to be no method of successfully depositing thermal spray materials of a wide variety of compositions on FRP in a production worthy manner.
It is an object of the present invention to provide a coating for fiber reinforced composite polymeric materials and components that are well bonded and have an exterior layer with high resistance to wear, corrosion or other unique properties not provided by the fiber reinforced composite materials.
It is further object of the present invention to provide a process for applying well bonded thermal spray coatings to fiber reinforced composite polymeric materials.
It is a particular object of the invention is to provide a thermal spray coating and a method of applying thermal spray coating to carbon fiber reinforced composite materials and components.